Now that the race is over, the least interesting thing about the president-elect, Barack Obama, may seem to be his skin colour. The same is true of Lewis Hamilton, Tiger Woods and even Boris Johnson. What really counts for us (and for them) now will be Iraq policy, how to sneak past a competitor or sink a hole in one, or how to make the transition from buffoon to statesman.

Even so, each of these multiracial heroes illustrates a deeper and longer-lasting truth about ourselves – that men (and women) are on the move, and are having lots of sex on the way. Kenya and Kansas; Grenada and Great Britain; English royals and Turkish politicians – Obama, Hamilton and Johnson all emerge from liaisons unthinkable until recently. In Tiger Woods’s term, the world is becoming Cablin-asian (a word he invented to describe his Caucasian, black, American Indian and Asian ancestry).

History has always been made in bed, but the beds are closer together. As a result, the human race is in the midst of a great averaging; and the future, more than likely, is brown.

Almost everywhere, the biological frontiers are becoming porous. An era of uniformity is at hand as hordes of people move in search of work or sunshine and, in the end, sex. In Britain, the proportion of the population born abroad has doubled in the past 50 years and is now 10%.

Intermarriage has been around for a long time. There have been Africans in Britain since the Romans, and by the 18th century these islands held 10,000 black people. Since then, the proportion of Britons who claim recent full or partial descent from Africa has gone up by 20 times and continues to climb. In 1991 more than 10% of citizens in one electoral ward in 10 were from an ethnic minority. Ten years later the figure was one in eight; and by 2011 the figure will rise to one in five.

All geneticists are firm believers in the healing power of lust; in the ability of desire to overcome social and geographic barriers. In 2001 about one British marriage in 50 – a quarter of a million in total, with many more couples cohabiting – was between partners from different ethnic groups. Hundreds of thousands of children have one parent from Britain and one from the Caribbean, and almost as many are the progeny of white and Asian parents. Such relationships are not, as often believed, to be found just among the poor, for more than half of these couples live in the suburbs and are richer and more educated than average.

So, assimilation is well under way in modern Britain, which is among the most sexually open nations in the world. Today, mate choice is made as much by level of education as by skin colour. Many other countries too, whether they like it or not, have opened up their gene pools.

For years, hurdles in the mind kept genes at home, as did the simple difficulties of travel. Some clues to the reason for the shift in our habits are obvious. How far was your birthplace from that of your partner, and how far apart were your mother and father, and your grandmothers and grandfathers, born? In almost every case, the distance has increased over the generations and continues to do so (my wife and I first saw the light 3,000 miles apart; my mother and father, about three).

The world’s sedentary habits are manifest in its surnames – each one a window into sexual history – as much as its genes. Until not long ago, a pedigree of names fitted well with national boundaries. The Camerons were more or less confined to Scot-land, and the Obamas to Kenya. Now names are on the move. In 1881 the Joneses were more or less confined to Wales, where in some villages they formed a majority. By 1998 my family had smeared itself across the Welsh borders, to northwest England and as far south as London.

No longer must a Jones marry a relative – as so many once did – for lack of choice. Instead they now come into contact with a diverse set of potential partners. The proportion of shared names in the marriage records of a typical English village has gone down by about 2% a year since the mid1970s.

One chromosome – the Y, carried by men alone – is transmitted in much the same way as surnames are: from father to son and to grandson. The Y also gives some hints about sexual habits of the past. In the United States about a fifth of all the genes in the African-American community are of European origin. But in that community almost a third of the genes on the Y chromosome originate from Europe: proof that there were many more cases of white men having sex with black women (often, no doubt, without the consent of the female involved) than the other way round.

Famously, President Thomas Jef-ferson had several children with his black slave Sally Hemings – and the evidence lies in the male chromosome of their descendants. That pattern of powerful men taking advantage of their position is also obvious across the whole of South America, with its enormously mixed populations, where once again many more genes were brought by European males than by European females.

In Britain the story is rather different. Many white Britons can trace a black ancestor from the small African population that lived in England several centuries ago. About half the men of a certain Yorkshire kindred, with the surname Revis, share a Y-chromosome type otherwise found only in west Africa. In the modern world too that pattern persists. British Afro-Caribbean males are half again as likely to marry a white female than are black women to find a white husband (while for Chinese people in a mixed-group marriage, those preferences are reversed).

Such long-distance matings are new. Most Sunday Times readers will see on their way to work tomorrow more people than the average member of the human species would until recently have seen in a lifetime. For 99% of our past, we were hunters and gatherers – rare and not particularly successful primates that gathered berries, hunted wild animals and flirted with extinction. At the DNA level we are far less different, one from another, than are chimpanzees – a strong hint that for most of history we lived in tiny, isolated groups, married the boy – or girl – next door and lost diversity as a result. If the few hunter-gatherer tribes still left a few decades ago were any guide, any attempt to join another group, let alone exchange genes with it, was likely to be met by death.

The great social change began – as most do – with economics; with the explosion in numbers that started as a result of farming. As the population boomed, people began to move, and biology soon reared its inevitable head, with the results we can see around us.

Obama has talked at length about the economy, about Main Street versus Wall Street and those with power versus those without. As politicians always do, he promises his country a better future – and he may succeed. To biologists, though, he is living proof that the future is almost here: a future that will look more or less like him.

Steve Jones is professor of genetics at University College London

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